This invention relates to a decurling operation and more specifically to a steam shower decurling system which is markedly more efficient and effective in achieving improved decurling capacity than traditional steam shower operations.
As is commonly known, when a web material is supplied in the form of a roll there is an initial curl in the web depending on such factors as the length of time the roll has been stored in stock and variation in the roll diameter, such as the curvature of the different convolutions making up the roll. In order to compensate for these influences on the straightness of a web, it has been found necessary to provide certain prescribed treatments to straighten the web material such as described in U.S. Pat. Nos. 3,185,616 and 3,649,447, so that when it is ultimately cut into sheets, the handling of the sheets may be facilitated and the undesirable curling effect eliminated. While these existing devices for straightening webs have been found useful, they have not been altogether satisfactory in the sense that they are expensive and/or require complicated mechanisms.
In the case of paper, it is known that increasing the moisture content of the paper will lead to less trouble from curling than paper having a lower moisture content. With respect to a coated sheet, for example, it is known that the latter will tend to curl towards the last side coated, and thus moisture is applied to the side opposite to which the sheet otherwise would tend to curl. Thus, as regards the paper industry, the controlled and uniform application of moisture to raw stock and/or coated webs is a goal of distinct importance.
Several methods have been proposed for applying moisture to a moving sheet or web. Examples of these include roll applicators such as the Dahlgren-type systems, electrostatic systems such as disclosed in U.S. Pat. No. 3,467,541 and steam shower devices. Other examples for adding moisture to a web include a method and apparatus for applying steam to and condensing moisture on a web that is backed by a heat conducting body, as disclosed in U.S. Pat. No. 2,370,811 and a device that directs a humid atmosphere towards the web, as disclosed in U.S. Pat. No. 3,238,635.
While these and other techniques have been found useful in applying moisture to a paper work piece so as to minimize the undesirable curling properties of the paper, there have been found inherent disadvantages in their use. In one instance for example, condensation of steam vapor on the web is produced because the web is backed by a heat sink or metal roll through which a cooling fluid is passed. In the case where humid atmosphere is directed toward the web the system has proven to be inefficient. Furthermore, in the case where steam showers are employed directly, decurling has been strictly governed by the steam pressure employed at the shower head. Lack of effective decurling lead to the utilization or reliance upon excessive steam pressure and/or reduced machine speeds. A change in pressure has been demonstrated to have an effect on other important variables such as the web temperature, web-to-shower distance and steam billowing. In addition, decurling regulated by steam pressure alone is uneconomical and inefficient. With a free floating web, a high level of exhaust is required to counter the rise in web-to-shower distance and to control the billowing effect resulting from the additional steam usage, the increased amount of steam required due to steam, which is essential to decurling, being drawn away by the exhaust. The resulting lack of decurling capacity leads to costly limitations on both quantity and quality.